Along with the increase in data traffic, high-speed communications such as 40 Gbps (gigabit per second), 100 Gbps, and the like per wavelength are being put to practical use in order to increase a capacity of an optical communication network. As a technique for realizing the high-speed optical communication, a technique of fabricating a fine optical waveguide structure with a silicon photonics technology has been drawing a attention. In the optical modulator of silicon photonics, a voltage to a PN junction formed at a center of the waveguide is applied, and an optical path length is varied by utilizing a change in a carrier density, that is, a change in a refractive index (carrier plasma dispersion). Since the change in the refractive index is proportional to the change in carrier density, it is desired to change the carrier density with a voltage change as small as possible. In the PN junction, when the bias voltage is applied in a forward direction, the change in the carrier density is larger and a modulation efficiency is better. However, there is a problem in that bandwidth is narrow. From a view at a driver side, a junction capacitance of an optical modulator acts as a load capacitance. A high frequency component signal attenuates and the band is degraded. In a case in which a pin diode is used, since the junction capacitance is larger, characteristics in a high frequency band become worse.
As a method of preventing band degradation by an optical modulator driven at high speed, a method of inserting a matching circuit formed by a capacitor (C) and a resistor (R) between a drive circuit and an optical modulator has been proposed (for instance, refer to Non-Patent Document 1). By inserting the matching circuit electrically equivalent to the PN junction of the optical modulator, the junction capacitance of the pin diode is reduced to a design capacity of the optical modulator.